As you may have noticed, all examples here use Object.defineProperty in preference to either __defineGetter__ or __defineSetter__, using the latter two only as fallback when the former is absent. While many browsers support these methods, not all do. Object.defineProperty is the future, and it is the standard; Microsoft has even gone on the record to say that they will not implement __defineGetter__ or __defineSetter__ in IE given the existence of the standardized method (props to them for that choice, by the way). For greatest forward compatibility with all browsers, you should use Object.defineProperty if it exists, and only fall back to __define{G,S}etter__ if it does not.
In a distant future we would like to remove support for __defineGetter__ and __defineSetter__, after ES5 adoption has taken off, so as not to distract from the standardized support. The less new web developers have to know about legacy extensions superseded by standardized alternatives, the better. This action is at least several years in the future, likely longer; being able to make the change will require preparation and adjustment in anticipation of that time.